Due to legal requirements, the light distributions of vehicle headlights must comply with a number of conditions. In addition to the legal requirements, customer requirements—for instance regarding homogeneity of a light distribution—must be implemented.
For example, as a matter of law, transitions from brightly lit to dimmed out distribution regions are defined as blurred light/dark boundaries (LD boundaries), wherein the LD borders must be neither too sharp nor too washed out—that is, the maximum sharpness (the degree of hardness of the LD boundary is specified by the measured value G) of the HD boundary is prescribed by law (in ECE Member States, lower boundaries for this measured value are also specified by law). Such a blurring of the LD boundary causes the LD boundary to be perceived by the driver as “softer”, and subjectively more pleasant.
The sharpness and/or blurring of this LD boundary is quantified by the maximum of a gradient along a vertical section through the LD boundary. For this purpose, the logarithm of the illuminance is calculated at measurement points in 0.1° intervals. Subtraction then gives the gradient function. The maximum of this function is referred to as the gradient of the LD boundary. Because this definition does not accurately model human perception of brightness, differently perceived LD boundaries may have the same measured gradient value and/or different gradients can be measured for similar-looking LD boundaries.
Another issue is the generation of segmented light distributions. These are used, by way of example, in the production of dynamic light distributions, for example in the case of a glare-free high beam. The technical field uses the term ‘ADB systems’ (Adaptive Driving Beam). In particular designs, such a dynamic light distribution is constructed from one or more individual light distributions. By way of example, individual light sources each generate a small segment in the light pattern, wherein an optical head is assigned to each of these individual light sources. The overlap of these light segments then produces the overall light distribution. Individual segments can be switched off (i.e., not illuminated) in the light pattern by switching off individual light sources. The segments in this case are typically arranged in rows and/or columns.
The use of different light modules to generate an overall light distribution as required by law can cause sharp transitions between the light distributions generated by the individual light modules, which are perceived as unpleasant. These transitions, or so-called inhomogeneities, become visible in front of the vehicle. Consequently, a light distribution is used which has very large gradients in the intensity transition, which are therefore not clearly perceived by the human eye.
One approach known in the prior art for softening the gradient is that of adjusting the curvature of the optical head (see, for example, DE 102 009 053 581 B3) to the extent permitted by the optical system (lens diameter, back focal distance of the lens). This approach is used in particular in the devices which are configured with an optical head. Such an adaptation can be achieved, for example, by the use of microstructures on the boundary surfaces of the imaging lenses, and is known from the prior art. With a change in curvature on the exit surface of an optical head, the strip-shaped light distributions, by way of example, are given variable size. As a result, a certain luminous flux fraction is distributed over a greater area. The result is a broadening of the LD region, whereby the human eye perceives the illumination transition with less contrast. However, this approach is of limited application. By way of example, the large gradient at the lower region of a segmented high beam light distribution—which will be discussed further below—cannot be manipulated in this manner.
In another approach known in the prior art, a roughening (for example, by sandblasting) which homogenizes transitions is undertaken on the optical head elements. The process of sandblasting always leads to different geometries in the tool or on the lens surface. The disadvantage of this approach is that each production batch looks different and leads to (mostly) small variations in the gradient values.
The solutions named above can therefore only be applied in special cases rather than generally (see, for example, DE 102 006 052 749 A1, DE 102 008 036 193 A1, EP 2518397 A2, DE 102 007 052 745 A1, DE 102 007 052 742 A1).
The disadvantages of the prior art described above should be remedied.